1. Field of the Invention
This invention relates to rails and in particular to rails exhibiting improved strength, hardness and toughness.
2. Description of Related Art
The problems with making rails for railways are well known and may be summarised as the difficulty of providing both a hard running surface together with a tough rail which in this technology means having a resistance to fracture. Treatments of the head to make it hard are well known, but in general are found to have corresponding deleterious effects on the toughness. The rail must be able to resist the propagation of fatigue cracks.
Modern high performance rails are currently made by rolling steel of an appropriate composition and then cooling it. The rail may be cooled either directly after leaving the rolling mill, perhaps having been reheated, or after subsequent heat treatment. Cooling is controlled and the object is to create pearlite as the main component of the rail head. This pearlite has particular qualities of hardness and the cooling rate is in fact controlled to be below a particular rate for the steel composition in question so that it passes into what is known as the perlitic area on the continuous cooling transition (CCT) diagram for the steel. In some cases the cooling may be particularly controlled so that the path on the CCT diagram to passes through what is known as the "perlitic nose" when a pearlite of a fine inter lamellar spacing and consequently higher strength and hardness is produced. Unfortunately modern rail technology is now approaching the limits of hardness that can be achieved by a perlitic head because of the reductions in toughness brought about by the processing for increased hardness.